Exploitation of potassium by various crop species from primary minerals in soils rich in micas

Abstract

We investigated the question of whether exchangeable K⁺ is a reliable factor for K⁺ availability to plants on representative arable soils (Aridisols) rich in K⁺-bearing minerals. Five soils with different textures were collected from different locations in Pakistan and used for pot experiments. The soils were separated into sand, silt, and clay fractions and quartz sand was added to each fraction to bring it to 1 kg per kg whole soil, i.e., for each fraction the quartz sand replaced the weight of the two excluded fractions. On these soil fraction-quartz mixtures wheat, elephant grass, maize, and barley were cultivated in a rotational sequence. Growth on the sand mixture was very poor and except for the elephant grass all species showed severe K⁺-deficiency symptoms. Growth on the mixture with silt and clay fractions was much better than on the sand fraction; there was no major difference in growth and K⁺ supply to plants whether grown on silt or clay, although the clay fraction was rich and the silt fraction poor in exchangeable K⁺. On both these fractions the plant-available K⁺ supply was suboptimal and the plants showed deficiency symptoms except for the elephant grass. This plant species had a relatively low growth rate but it grew similarly on sand, silt, and clay and did not show any K⁺ deficiency symptoms, with the K⁺ concentration in the plant tops indicating a sufficient K⁺ supply regardless of which soil fraction the plants were grown in. The reason for this finding is not yet understood and needs further investigation. It is concluded that on soils rich in mica, exchangeable K⁺ alone is a poor indicator of K⁺ availability to plants and that mica concentrations in the silt and clay fraction are of greater importance in supplying crops with K⁺ than exchangeable K⁺.